Accounts of agarwood
distillation vary from source to source as we will see. Baruah et al.
(1982) distinguished four grades of infected fragrant wood: the poorest
grade being buff-coloured (Dhum) and being exclusively used for
distillation, better grades being used to make incense agar-battis.
Chaudhari (1993) named these as:

Grade 1 Black/True
Agar: mainly exported to Arabia as incense

Grade 2 Bantang:
mainly exported to Arabia as incense

Grade 3 Bhuta or Phuta: sometimes extracted for a superior oil

Grade 4 Dhum: used for oil

Chaudhari also estimated the
amount of wood available in India as 31 metric tons, which would yield
77.5Kg agarwood oil on a 0.25% yield basis, naming the stills in the Naga
Hills as being at Manipar Road (3 stills), Barapathar (27 stills) and
Dhansari (now shut down).

An 80 year-old tree can yield 6-9 Kg of agarwood
oil (Sadgopal 1960), although Mahindru (1992) puts the figure at 2.7 to 3.6
Kg/tree for 50-year-old trees in India, and Gianno (1986) puts it even lower
at 1kg per tree with a girth of above 20cm dbh.

The fragrant oils that result
from the individual field still distillations are classified as follows: ­
primary grade oil, usually a darker colour originating from the more heavily
stained­ wood sections, which incidentally are often associated with the
­pathogenic fungus Philophora parasita. Secondary grade oil is
­usually lighter in colour, often originating from pale to ­gray-stained
fungal infected sections. The spent wood after distillation is dried and
often used for making agar-battis, although processes are reported for
extracting oil from the spent wood.

The Chakrabarty K et al.
(1994) describe the distillation of Agarwood oil in great detail in the
towns of Hojai, Nilbagan and Islamnagar in the Naogaon district of Assam as
a result of the Traffic investigations of 1993, speculating that 500-1000
tolas a month of oil (1 tola = 46.48g) are produced in Hojai, legally
obtained wood being supplemented by that illegally obtained from India and
Bhutan. The report also mentions distillation in Manipur in Thambu Bazar,
and pinpoint Bombay as the production centre for agarwood attars. Finally
the authors identify Myanmar and Tuensang in Nagaland as a major smuggling
points.

Wood from felled trees is cut
into pieces and thrown into water – those pieces containing more
than 25% oleoresin, will sink. Infected areas of wood are carefully scraped out with a special tool
(called a batali). After sorting pieces into grades, the selected pieces
are ground and soaked in water (often in a drum container) for 48 hours,
although other reports say 1-5 days. The wood is the powdered manually with
a dhenki. Distillation is carried out for 5-6 hours with cohabation of the
distillation waters, although 24-48 hours is reported elsewhere. Chakrabarty
et al. (1994) suggest 8-12 Kg of wood in 80 litres of water are
distilled for 3 days, describing a cohabation arrangement, although it also
describes an improved process developed at the Regional Research Laboratory,
Jorhat in Assam which uses a stainless steel still and yields a clear oil
free from metallic impurities.